1. Field of the Invention
The present invention relates to a screen printing method and a screen printing machine each of which is for printing a print material on an object through pattern holes of a print mask.
2. Discussion of Related Art
A screen printing method and a screen printing machine are widely used in various industrial fields such as an electric-circuit production industry. For example, when an electric circuit is produced, solder paste is printed on a circuit substrate (e.g., a printed wiring board, PWB) on which the electric circuit is to be assembled. After one or more electric components (ECs) are mounted on the circuit substrate, the circuit substrate is heated to melt the solder paste printed thereon and thereby electrically connect the ECs to the circuit substrate. In this field, it is practiced to form solder bumps on a circuit substrate by first printing solder paste thereon and subsequently heating and melting the printed solder paste.
There are known two major methods of printing solder paste on circuit boards. One of them is an off-contact-type screen printing method, and the other is a contact-type screen printing method. In the off-contact method, a small space is left between a print mask and a circuit substrate, and a plate-like squeegee is moved on a surface of the print mask while locally pressing the mask against the circuit substrate and moving solder paste along the surface of the mask, so that while the mask contacts the substrate, the solder paste is printed on the substrate through pattern holes of the mask. Generally, a metal mask that is formed of a metal such as copper, stainless steel, or nickel is suitable for printing of a highly viscous fluid such as solder paste. However, the metal mask is too rigid to be used in the off-contact printing. Therefore, generally, a so-called combination mask including a metallic portion and an elastically deformable mesh portion surrounding the metallic portion is used in the off-contact printing. In the off-contact printing, the print mask, except for a local portion thereof that is currently pressed by the squeegee against the circuit substrate for printing the solder paste, is kept away from the substrate, and the local portion currently pressed by the squeegee is separated, little by little, from the substrate after the squeegee has passed over the local portion. Thus, the solder paste printed on the circuit substrate is advantageously separated from the pattern holes of the print mask.
However, when the solder paste placed on the upper surface of the print mask is forced into the pattern holes of the mask by the plate-like squeegee, so as to be printed on the circuit substrate through the holes of the mask, the. solder paste may enter some holes of an adjacent portion of the mask that is adjacent to the local portion currently pressed by the squeegee against the substrate and is currently away from the substrate, and may spread on the lower surface of the mask. In this case, the solder paste may be applied to other spots on the circuit substrate than prescribed print spots thereon, or may be. adhered to the lower surface of the mask to soil the same. These phenomena do not lead to accurate printing.
In this background, currently, the contact-type screen printing method is widely used. In the contact-type method, solder paste is printed in the state in which a print mask is held in close contact with a circuit substrate. This method is particularly suitable for printing of a highly accurate and fine pattern. Either a combination mask or a metal mask may be employed as the print mask. Generally, this method needs a special technique for separating the print mask and the circuit substrate from each other after the solder paste is printed on the substrate. There are known various separating techniques.
However, in. the contact-type method, the print mask and the circuit substrate are kept in close contact with each other while the solder paste is printed on the substrate. Therefore, the solder paste may enter, because of its capillary phenomenon and/or surface tension, the interface between the mask and the substrate. Meanwhile, in the case where the circuit substrate is sucked, by vacuum, against a support surface of a substrate supporting device, air present between the mask and the substrate are sucked by the vacuum via the through-holes of the substrate, so that the solder paste may be sucked into the interface between the mask and the substrate. Moreover, the solder paste may run on the lower surface of the print mask, because of small vibrations of the substrate supporting device. These phenomena do not result in accurate printing.
It is currently practiced to clean the upper and lower surfaces of the print mask, for solving the above-described problems. There are two cleaning methods, one is a wet method in which the solder paste is solved by a solvent and then is cleaned by a cleaning paper, and the other is a dry method in which the solder paste is just cleaned by a cleaning paper. In some cases, both the wet and dry methods are employed. In either method, it is difficult to clean up the solder paste completely. The most important problem with the cleaning methods is that the cleaning of the print mask results in increasing the cycle time and accordingly lowering the production efficiency. In addition, the expensive solder paste cleaned off the print mask cannot be recycled, which is against recent tendency toward environmental pollution control. Moreover, an exclusive cleaning device is needed, which leads to increasing the running cost.
Another problem with the contact-type printing is about the separation of print mask and circuit substrate. It is not easy to separate the print mask closely contacted with the circuit substrate, from the circuit substrate, without damaging the shape of the solder paste printed thereon. The degree of ease of separation of highly viscous fluid (i.e., solder paste) from pattern holes of print mask largely depends on an aspect ratio, h/f, as a ratio of an area, h, of side surface of each pattern hole to an area, f, of print pattern of the mask. Generally, it is preferred that the aspect ratio h/f be not greater than a solder-print critical aspect ratio (h/f=0.4 to 0.5). In addition, the printability of solder is influenced by various factors including the design or composition of solder paste, the machining or material of print mask, the circuit substrate, the printing machine, and the operator. It is almost impossible to perform printing most efficiently under the best conditions of those factors. Therefore, there is a need to obtain such a special technique which can compensate for some inevitable defects of those factors. For example, a servo system is used to separate the print mask and the circuit substrate from each other by moving them over a predetermined distance at empirically obtained speeds and accelerations. However, it is impossible to move, at a certain speed or acceleration, the circuit substrate with respect to its entirety, because of the elasticity of the print mask. That is, the central portion of the circuit substrate is moved away from the print mask, faster than the end portions of the same are. It is difficult to construct a reliable multidimensional space-analysis model to control drive shafts of the servo system used to separate the print mask and the circuit substrate from each other. Thus, currently, it is difficult for the contact-type printing method to perform a sufficiently stable printing for a long time. The properties of the solder paste change day by day. Generally, the basic data of the solder paste, such as viscosity, may change by xc2x115% a day. Now, a huge data base including data about various variable factors is under construction in order to establish useful conditions for the separation of print mask and circuit substrate, so that the data base is utilized by a computer. However, this function is not a basic function which is properly required for the printing machine. In addition, the printing machine needs a complex device for the separation of print mask and circuit substrate, which leads to increasing the production cost thereof.
Although the above description relates to the case where solder paste is printed on circuit substrates, the same problems as identified above may occur to screen printing methods and screen printing machines which are used in other industrial fields.
The present invention provides a screen printing method and a screen printing machine which have one or more of the following technical features that are described below in respective paragraphs given parenthesized sequential numbers (1) to (10). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter feature. However, the following technical features and the appropriate combinations thereof are just examples to which the present invention is by no means limited. In addition, in the case where one technical feature recites a plurality of items, it is not essentially required that all of those items be simultaneously employed. That is, it is possible to select and employ only a portion (one, two, . . . , but not all) of those items.
(1) According to a first feature of the present invention, there is provided a method of printing a print material on a surface of an object by placing the print material on a surface of a print mask having a plurality of holes, positioning the print mask relative to the object such that the print mask is near, and substantially parallel, to the surface of the object, moving at least one of a squeegee and a first combination of the print mask and the object relative to the other of the squeegee and the first combination, in a print direction along the surface of the print mask, while locally pressing, with the squeegee, the print mask against the surface of the object and pressing, with the squeegee, the print material along the surface of the print mask, so that a portion of the print material is printed on the surface of the object through the holes of the print mask, the method comprising the steps of locally pressing, with a mask-press member, the print mask against the surface of the object, at a position distant from the squeegee by a predetermined distance which assures that the mask-press member does not contact the print material being pressed by the squeegee, and moving at least one of a second combination of the squeegee and the maskpress member and the first combination of the print mask and the object relative to the other of the second combination and the first combination, in the print direction, such that the maskpress member precedes the squeegee in the print direction. The term xe2x80x9cnearxe2x80x9d means that the print mask is close to the surface of the object but is not in contact with the same, and means the presence of a distance between the mask and the object that allows the squeegee or the mask-press member to locally press the mask against the object and cause a portion of the mask to contact the object.
The present printing method is a sort of screen printing method. In the present screen printing method, the squeegee prints the print material on the object through the hole or holes (xe2x80x9cpattern hole or holesxe2x80x9d) of a portion of the print mask that is currently pressed against the object by the mask-press member. Thus, the present printing method enjoys the aboveindicated advantages of the contact-type printing method. In addition, each portion of the print mask over which the squeegee has passed is quickly peeled off the surface (xe2x80x9cprint surfacexe2x80x9d) of the object, while being somewhat inclined relative to the print surface. Thus, the print material printed on the object is easily separated from the pattern holes, and accordingly the present printing method enjoys the advantages of the off-contact printing method. That is, the present printing method enjoys the advantages of both the contact-type and off-contact-type printing methods, while avoiding the disadvantages of the same. Thus, it can perform excellent printing at low cost. Here, it is noted that it is possible to employ, in addition to the mask-press member preceding the squeegee, another mask-press member following the squeegee. In the latter case, each portion of the print mask over which the squeegee has passed is peeled off the print surface of the object, after the following mask-press member has passed over the each portion. In contrast, in the case where the following mask-press member is not employed, each portion of the print mask over which the squeegee has passed is immediately peeled off the print surface of the object.
(2) According to a second feature of the present invention that includes the first feature (1), the mask-press member comprises a press roller which rolls on the surface of the print mask.
Alternatively, the mask-press member may be a slide member which is slideable on the print mask. In the latter case, although the overall construction of the printing machine may be made simpler, a greater friction force is produced between the slide member as the mask-press member, and the print mask, which may disadvantageously lead to causing a movement of the mask out of position relative to the object. In contrast thereto, if the press roller is employed as the mask-press member according to the second feature (2), only a small friction resistance is produced between the press roller and the print mask, which minimizes the movement of the mask out of position relative to the object. Meanwhile, since the squeegee must slide on the print mask, the friction force produced between the squeegee and the mask provides some force to move to mask relative to the object. However, since the press roller presses the mask against the object, the friction force produced between the roller and the mask resists the force, exerted by the squeegee, to move the mask relative to the object. The pressing force of the press roller provides the friction force, produced between the print mask and the object, which is greater than the rolling friction force produced between the roller and the mask. Therefore, the movement of the mask out of position relative to the object is resisted by the force equal to the difference between the friction force produced between the mask and the object and the friction force produced between the mask and the roller. If this resisting force is greater than the friction force produced between the squeegee and the print mask, the mask is never moved relative to the object; and even if the resisting force is smaller than the friction force, the amount of movement of the mask relative to the object can be effectively reduced.
(3) According to a third feature of the present invention that includes the first or second feature (1) or (2), the object comprises an electric-circuit substrate and the print material comprises a solder paste.
(4) According to a fourth feature of the present invention, there is provided a printing apparatus, comprising a supporting device having a support surface which supports an object; a mask holder which holds a print mask having a plurality of holes, such that the print mask is near, and substantially parallel, to the object supported by the support surface of the supporting device; a squeegee having a press surface which locally presses the print mask held by the mask holder against the object supported by the support surface; a moving device which moves, while the squeegee locally presses the print mask against the object supported by the support surface, at least one of the squeegee and a first combination of the supporting device and the mask holder relative to the other of the squeegee and the first combination, in a print direction along the support surface, so that the squeegee presses a print material placed on a surface of the print mask, along the surface of the print mask, and forces a portion of the print material into the holes of the print mask; a mask-press member which locally presses the print mask against the object, at a position distant from the press surface of the squeegee by a first predetermined distance which assures that the mask-press member does not contact the print material being pressed by the squeegee; and a distance maintaining device which maintains, when the moving device moves, in the print direction, at least one of a second combination of the squeegee and the mask-press member and the first combination of the supporting device and the mask holder relative to the other of the second combination and the first combination, the first distance by which the mask-press member precedes the squeegee in the print direction.
The present printing apparatus is a sort of screen printing machine. The present screen printing machine can advantageously carry out the screen printing method according to the first feature (1). It is preferred that the squeegee be one which has a press surface which is inclined relative to the print mask, forward in the print direction, and which is slid on the mask while pressing the print material (e.g., solder paste) along the surface of the mask and forcing, owing to its wedge effect, the print material into the holes of the mask.
(5) According to a fifth feature of the present invention that includes the fourth feature (4), the mask-press member comprises a press roller which rolls on the surface of the print mask.
(6) According to a sixth feature of the present invention that includes the fourth or fifth feature (4) or (5), the moving device comprises a movable member which holds at least one of the squeegee and the mask-press member; and a movable-member moving device which moves the movable member in the print direction parallel to the support surface of the supporting device that supports the object.
Alternatively, the moving device may be one which moves the supporting device and the mask holder, relative to the squeegee and the mask-press member. In the latter case, however, a wide space is needed which allows the supporting device and the mask holder to be moved relative to the squeegee and the mask-press member, and which leads to increasing the overall size of the screen printing machine. In contrast thereto, the screen printing machine according to the sixth feature (6) moves the squeegee and/or the mask-press member relative to the supporting device and the mask holder, such that the squeegee and/or the press member are moved in a space provided above the supporting device and the mask holder. Thus, the size of the present machine need not be increased.
(7) According to a seventh feature of the present invention that includes the sixth feature (6), the printing apparatus further comprises an elevating and lowering device which elevates and lowers the squeegee relative to the movable member, and thereby moves the squeegee in a first direction toward, and in a second direction away from, the support surface of the supporting device.
According to an eighth feature of the present invention that includes the sixth or seventh feature (6) or (7), the printing apparatus further comprises an elevation and lowering device which elevates and lowers the mask-press member relative to the movable member, and thereby moves the mask-press member in a first direction toward, and in a second direction away from, the support surface of the supporting device.
In the case where the squeegee and the mask-press member are supported by a single elevator member, the squeegee and the mask-press member may be elevated and lowered by a common elevating and lowering device. Otherwise, the squeegee and the mask-press member may be elevated and lowered by respective exclusive elevating and lowering devices, independent of each other. The latter manner is more useful than the former manner. For example, the print material which has been moved by the squeegee to one of opposite ends of the print mask in the print direction can be easily returned to the other end of the mask, irrespective of whether printing is effected during the returning of the print material. In addition, the respective contact forces exerted to the print mask by the squeegee and the mask-press member can be adjusted or controlled, independent of. each other.
(9) According to a ninth feature of the present invention that includes any one of the fourth to eighth features (4) to (8), the printing apparatus further comprises a following press member which is different from the mask-press member as a preceding press member, which is distant from the squeegee by a second predetermined distance in a direction opposite to the print direction, and which locally presses the print mask against the object supported by the support surface, the first combination comprising the following press member in addition to the squeegee and the preceding press member.
Since the present screen printing machine includes both the preceding and following press members, the squeegee has only to print the print material at the central portion of a portion of the print mask that is currently pressed by the two press members. Thus, the present machine can perform a more stable printing. For example, in the case where only the preceding press member is employed, the squeegee is required to not only force the print material into the holes of the print mask but also press the mask against the object. In the latter case, therefore, if a suitable contact force, exerted by the squeegee to the print mask, for forcing the print material into the pattern holes is smaller than a suitable force for pressing the mask against the object, the former contact force must be sacrificed. In contrast thereto, the screen printing machine according to the ninth feature (9) is free of this problem. Meanwhile, if both of the preceding and following press rollers are provided by press rollers, a greater friction force is produced between the print mask and the object, which contributes to preventing the mask from being moved out of position relative to the object.
(10) According to a tenth feature of the present invention that includes any one of the fourth to ninth features (4) to (9), the supporting device comprises a substrate supporting device which supports an electric-circuit substrate as the object, and the squeegee prints a solder paste as the printing material, on the electric-circuit substrate supported by the substrate supporting device.